Generally, a bicycle derailleur is provided with a base member, a movable member movable axially of the multistage sprocket assembly, a shifting means operated from the exterior to move the movable member axially of the same, a guide mechanism for guiding the driving chain from one sprocket to another at the same, and a return spring. The derailleur, when a control wire is pulled for changing the bicycle speed, moves the movable member forwardly against the return spring, and when the control wire is slackened, moves the movable member backwardly by virtue of a restoring force of the return spring, thereby shifting the driving chain from one sprocket to another at the sprocket assembly.
The return spring comprises a coiled spring whose both ends extend tangentially outwardly of its coiled portion. The return spring is supported at is coiled portion around a pivot shaft connecting a linkage member at the shifting means with either the base member or movable member. The return spring abuts at one end against the base member or movable member, and is twisted to contract the coiled portion in diameter so that the spring deflects as a whole. Thereafter, the spring contacts at the other end elastically with the linkage member.
In such conventional deraillieur, a torsion stroke necessary to mount the return spring deflected and an initial load to be given to the shifting means are inevitably defined, so that the return spring is designed to have a spring constant corresponding to the torsion stroke and initial load. As a result, the characteristic of the return spring is specified so as to increase a variation in the spring force with respect to the stroke or length of movement of the movable member, whereby the restoring force of the spring becomes intensive as the number of speed change stages increases. Accordingly, the restoring force of the return spring, when the chain is shifted to the largest diameter sprocket at the multistage sprocket assembly, is very large, whereby a control lever requires a larger operating force for deflecting the return spring for changing the bicycle speed. Hence, the conventional derailleur has been inferior in speed change efficiency.